Interpretive Summary: Ich is a common parasite of fish and is found throughout the world. It appears as small white spots the size of a pin head. Ich can be a devastating problem in aquaculture and can be passed to nearby facilities through poor management practices. A chemical called malachite green was very effective at controlling Ich, but it has been found to cause cancer and is therefore illegal to use. A relatively new therapeutant called peracetic acid (PAA) was tested to control Ich on fish. Carp were infested with Ich and then continuously exposed to PAA for 4 days. Results show that PAA initially eliminated all the Ich, but it came back. Therefore, concentrations of PAA may have been to low. These results are very promising and further research will be done.

Technical Abstract:
The parasitic ciliate Ichthyophthirius multifiliis infests all species of freshwater fish and can cause severe economic losses in fish breeding. The most effective treatment, malachite green, has been banned in Europe and North America for use in food-fish production. Peracetic acid (PAA) was found to be toxic to I. multifiliis theronts at low concentrations. I. multifiliis-infested carp were exposed to 1 ppm PAA in a dynamic exposure by means of peristaltic pumps. Five days after infestation, gills, tail fins and skin below the dorsal fin were observed microscopically for I. multifiliis abundance. After PAA exposure, PAA-treated fish showed lower infestation of I. multifiliis in all investigated tissues than the unexposed control fish. The infestation increased in the control group whereas the infestation in the PAA-exposed groups significantly decreased (p=0.0083, Bonferroni correction). The fish in the two exposure groups showed a slight reinfestation with I. multifiliis. This might be caused by a peroxide degradation (hydrolysis) and/or reduction of the delivered PAA concentration. Thus, PAA concentrations were possibly too low to be effective on the released trophonts and/or the infective theronts. This hypothesis is corroborated by the fact that the I. multifiliis in the gills, skin, and fins of the PAA-exposed carp were in an early developmental stage.